OnePlus software tends to be among the best for Android devices. Its update process ... not so much, at least as of late. The OnePlus 7T, for example, had to wait half a year to get its Android 11 update, and some carrier variants took even longer. If you want to try out a new flavor of Android on your OnePlus 7T, now you can: there's an official build of the LineageOS custom ROM available for it.
This build is Lineage 18.1, which is based on Android 11. So it'll have all the bells and whistles you'd expect from a modern, updated phone, minus some of OnePlus's user interface tweaks from the Oxygen skin. (Though to be fair, a lot of the same tweaks are in place in Lineage — good software engineers copy, great software engineers steal.) The build is a "nightly" release from Lineage, which means you should expect some bugs, though Lineage's nightlies tend to be more stable than that term implies elsewhere. More reliable builds may or may not be in the pipeline.
To install it, you'll need to unlock the bootloader on your OP 7T, install a custom recovery, then use that to install the ROM itself and erase the factory software. You'll also probably want a build of Gapps to flash, assuming you want access to the Play Store and other Google software. While this is a fairly straightforward process for old hats, if you're new to Android ROMs, don't worry: a little Google searching will set you right.
Thanks to enthusiastic users and reliably open bootloaders, OnePlus devices are a frequent recipient of ROM builds from the official Lineage team (and open source builds from the community at large). Just yesterday the older OnePlus 6 got a Lineage build, following the OnePlus 6T, both of which have yet to see Android 11 from their official masters.